The invention relates to a method for manufacturing a wire cathode covered with an emitter material, in which method a material is deposited on a wire by means of electrodeposition.
The invention also relates to an apparatus for depositing an emitter material on a wire, said apparatus comprising a means for holding the wire, a drop holder comprising a suspension and means for applying an electric potential to the wire and the drop holder.
Wire cathodes are used for instance in flat electroluminescent picture display devices such as flat CRTs, or in lamps.
It is known to manufacture wire cathodes by means of a method in which an emitter material is deposited on a wire by means of electrodeposition. Such a method is known, e.g. from German Patent No. 874,337. In said method, a wire cathode wound on a reel is passed through a suspension in which an alkaline-earth metal compound is dispersed, which compound is electrodeposited on the wire. To enable electrodeposition, the wire is used as a cathode, while a pipe of metallic material, which is connected to a supply pipe to continuously provide the pipe with the suspension, is used as an anode. The cylindrical pipe forms an electrophoretic cell through which the wire is led. The movement of the cathode wire through the electrophoretic cell is temporarily interrupted at the sections of the wire that need to be coated whereupon the polarisation voltage is switched on and the electrophoretic process is started. Once the cathode wire is provided with the desired coating, the polarisation voltage is switched off again and transport of the wire is resumed until the next section of the wire to be coated is positioned in the electrophoretic cell. Eventually, the coated cathode wire is wound on a second reel.
A disadvantage of the known method is that undesired deposits are obtained on those parts of the cathode wire that need not be coated. In operation, this leads to electron emission on parts of the wire where it is not desired. Furthermore, soldering of the wire to connection means is adversely affected by such unwanted deposits.
It is an object of the invention to provide a method for depositing an alkaline-earth metal compound on a wire cathode in which one or more of the above cited problems are obviated or at least alleviated.
For this purpose a method of the type described in the opening paragraph is characterized in that a suspension comprising the material is transferred by a drop holder, which is positioned around the wire, by movement in a direction transverse to a longitudinal axis of the wire, whereafter an electric voltage is applied to the drop holder and the wire to deposit the emitter material on the wire, after which the drop holder is withdrawn from the wire again.
In the known method it is required that all parts of the wire pass through the electrophoretic cell, including those parts which need not be coated. This implies that all parts of the wire are in contact with the suspension for a certain period of time, leading to undesired deposits on those parts of the cathode wire that need not be coated. Apart from the above-mentioned problem of unwanted deposits, this also leads to an early depreciation of the suspension or unwanted changes in the composition of the suspension requiring frequent renewal of the solution.